diff --git a/IOTeensy.cpp b/IOTeensy.cpp
index 18d9d4d..8f8ac27 100644
--- a/IOTeensy.cpp
+++ b/IOTeensy.cpp
@@ -87,7 +87,7 @@ void CIO::startInt()
   ADC0_SC2   = ADC_SC2_REFSEL(1) | ADC_SC2_ADTRG;                     // Voltage ref internal, hardware trigger
   ADC0_SC3   = ADC_SC3_AVGE | ADC_SC3_AVGS(0);                        // Enable averaging, 4 samples
 
-  ADC0_SC3   = ADC_SC3_CAL;
+  ADC0_SC3  |= ADC_SC3_CAL;
   while (ADC0_SC3 & ADC_SC3_CAL)                                      // Wait for calibration
     ;
 
@@ -108,7 +108,7 @@ void CIO::startInt()
   ADC1_SC2   = ADC_SC2_REFSEL(1);                                     // Voltage ref internal, software trigger
   ADC1_SC3   = ADC_SC3_AVGE | ADC_SC3_AVGS(0);                        // Enable averaging, 4 samples
 
-  ADC1_SC3   = ADC_SC3_CAL;
+  ADC1_SC3  |= ADC_SC3_CAL;
   while (ADC1_SC3 & ADC_SC3_CAL)                                      // Wait for calibration
     ;